This study proposes to investigate the evolutionary consequences of sustained existence at high and low population densities. The study will center on six populations of Drosophila melanogaster, three which have been kept at high densities (K-selected) and three kept at low densities (r-selected). These populations have been evolving under these conditions for over five years (about 100 generations). Changes in density-dependent rates of population growth, density-dependent survival and female size have already been documented for these populations. The present study will investigate (1) differences between r- and K-selected populations in relative competitive ability and efficiency of food utilization; (2) the possibility that heavier K-selected females have higher fecundity and that increased larval viability may be correlated with increased size; (3) the location of genes responsible for increased larval survival and female adult size and the genetic correlation between these traits. The results from this study would increase our understanding of the effects of density on natural populations and have a number of potential applications relevant to human health, such as predicting the reproductive potential of disease vectors.